We are using genetic and molecular biological approaches to analyze endogenous murine leukemia viruses of BALB/c and AKR mice. Analysis of the virion proteins and the genomes of the viruses and of recombinants between them allows one to obtain physical genetic maps of the murine leukemia virus genome. These studies have let to the identification of regions of the viral genome that determine host rang (N- or B-tropism; envelope glycoprotein).